a Sulphate Attack on Concrete Slabs

Olympic Construction Investigate a Sulphate Attack on Concrete Slabs

Olympic Construction Olympic Construction News, Uncategorised

Olympic Construction visited a property this week where the installed concrete slab had heaved. The upward movement of a slab had been caused by underlying soils and contaminated infill, expanding, and reacting with the concrete. This is commonly termed as a Sulphate attack.
In principle, the scheme has been rolled out with good intentions. It has been designed by civil servants, regulators, and energy/insulation advisors. This is a group of well motivated experts who are working under intense pressure to turn an emergency job creation scheme into something that delivers energy savings, eliminates corruption and doesn’t result in irreparable damage to your home.
a Sulphate Attack on Concrete Slabs
Photograph showing wet cavity wall insulation.

Sulphate salts in the form tested ‘Grey granular’ are likely to be from contaminated infill used to form the floor slab. The problem of sulphate affected floors, is caused by the use of industrial waste materials, used as hardcore infill under concrete floors in domestic properties.
The three main infill materials of a solid floor to a property of this age were Red Ash (Shale), Black Ash, Slag, and Grey Fly Ash. Other industrial materials and building rubble could also be used and present a potential problem.
These materials originated from various industrial sources such as Coal Mines, Steelworks, Foundries, Power Stations, and so on. All of which were present in the Northwest of England. As this material was widely available and was virtually free, builders used the material extensively as its use at that time did not contravene any Building Regulations.
It became apparent that this infill material could cause damage to the concrete floors and masonry below DPC level of the property, due to the sulphate present in these materials. The sulphate in the hardcore infill when combined with moisture (water) can cause the sulphate to migrate into the concrete floor slab, causing cracking and upward movement of the floor slab and, in extreme cases, cracking and lateral movement of the masonry below Damp Course level.
The Building Research Establishment (BRE), started issuing information on this problem in the mid-1950s and in 1967 the Building Regulations changed to restrict the use of industrial materials as hardcore infill. At the same time a 500g polythene membrane was introduced to provide a barrier between the concrete floor slab and the hardcore infill. This membrane was increased in thickness to l000g in 1970 and to 1200g in the early 1980s.
a Sulphate Attack on Concrete Slabs 2
Photograph showing condensation on a surface at dew point

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